JEE MAIN - Physics (2019 - 10th April Evening Slot - No. 25)
A 2 mW laser operates at wavelength of 500 nm. The number of photons that will be emitted per second is :
[Given Planck's constant h = 6.6 × 10–34 Js, speed of light c = 3.0 × 108
m/s]
5 × 1015
1.5 × 1016
1 × 1016
2 × 1016
Explanation
$$2 \times {10^{ - 3}} = {{hc} \over \lambda }{{dn} \over {dt}}$$
$${{dn} \over {dt}} = {{2 \times {{10}^{ - 3}}\lambda } \over {hc}}$$
$$ = {{2 \times {{10}^{ - 3}} \times 500 \times {{10}^{ - 9}}} \over {6.6 \times {{10}^{ - 34}} \times 3 \times {{10}^8}}}$$
$$ = {{1000} \over {6.6 \times 3}} \times 10 = 5 \times {10^{15}}$$
$${{dn} \over {dt}} = {{2 \times {{10}^{ - 3}}\lambda } \over {hc}}$$
$$ = {{2 \times {{10}^{ - 3}} \times 500 \times {{10}^{ - 9}}} \over {6.6 \times {{10}^{ - 34}} \times 3 \times {{10}^8}}}$$
$$ = {{1000} \over {6.6 \times 3}} \times 10 = 5 \times {10^{15}}$$
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